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I. Nuclear Magnetic Resonance

VIII. NMR Scope Program

IX. Pulsed NMR

X. Pulsed NMR Equipment

XI. Pulsed NMR Setup

XII. Pulsed NMR Appendices

Check that the Equipment is set to the following settings:

PRE-Amp SRS SR560

• Input - A, DC
• LF Roll Off - 0.3Hz
• Gain - 2K (or less watch out for the overload light)
• RF Roll Off - 10KHz

10LA Amplifier:

• This is the RF power amplifier
• Power - FWD

Fluke Frequency meter:

• Input - Channel A, Slope +, AC, Atten X1
• Function - freq. A
• Mode - cont.

5110B Synthesizer Driver:

• Frequency standard - internal
• Circuit check - 12.6 VDC
• Set to operate

5105 Frequency Synthesizer:

• Circuit check - 5110B
• Frequency selection - local keyboard, operate mode
• Search control - local
• Power - on
• Output - on the blue dot
• Input: we want the resonant frequency, 16.54xxx. On the keypad you will see three sections: Megahertz (not marked), Kilohertz, and Hertz. For example, to input 16.5435 MHz, press from left to right 0 1 6 5 4 3 S 5 5 5, where S stands for search and allows you to search for a more precise resonant value by rotating the knob labeled "search oscillator."

Tektronix 2230 Digital Oscilloscope

• Input - use either Channel A or B
• A Trigger - (i) use EXT : input from TO of the 535 Delay/Pulse Generator; (ii) use NORM; (iii) for A&B INT, use Channel 1; (iv) for A source : use EXT; (v) A EXT Coupling : use AC. To see the waveform easier, select the STORE mode using the button STORE/NON STORE next to the buttons of ACQUISITION at the top of the oscilloscope.
• Nice settings to start with: 5V/div, 1ms/div

DG535 Four Channel Digital Delay/Pulse Generator:

• Read "Using the SRS Model DG 535 Delay/Pulse Generator in the Pulsed NMR experiment" before proceeding.

Using the SRS Model DG 535 Delay/Pulse Generator in the Pulsed NMR experiment:

We use the DG 535 (henceforth the "pulse generator") to generate voltage pulses of arbitrary duration and timing to switch on and off the 16.5 MHz signal going to the RF power amplifier. The duration and timing of these bursts of radiofrequency radiation determines the behavior of the nuclear spins.

Find the Menu section on the front panel of the pulse generator. Press the Trigger button. You will see a menu that looks something like this:

Int Ext Ss Bur Line

The underscore under the I in Int is the cursor, which you can move with the left and right arrows on the front keypad. If the cursor is not under Int, move it so that it is. When on the Int setting, the pulse generator will send a step to the trigger output (T0) at a rate determined by an Internal clock. Press Trigger again. You will see a menu that looks something like this:

Rate = 6.000

This is the rate at which trigger pulses are sent on the T output in pulses per second. Now the cursor indicates which of the displayed digits you are editing. Use the right and left arrows to move the cursor, and the up and down arrows to change the digit values.

Press the Delay button. You will see a menu that looks something like this:

A = T + 0.008 000 000 000

This means that 8 ms after each trigger pulse, the pulse generator will send a step to the A output. Press Delay again. You will see a menu that looks something like this:

B = A + 0.000 055 000 000

This means that 55 microseconds after the step is sent to A, a step will be sent to B. The 'A +' means that the B output is delayed relative to A. You could change this so that B is delayed relative to T (or C or D) by editing the A just like you would edit the numbers.

Continuing to press Delay allows you to set the delays for channels C and D. Note that C should be timed relative to A, and that D should be timed relative to C.

In addition to the step outputs A,B,C and D, there are four pulse outputs. The output labeled AB puts out a high voltage only for the period between when a step is sent to A and when a step is sent to B. I.e., with the settings above, it turns on at time T+8ms and turns off 55 microseconds later. Channel AB is the inverse of AB. The CD pulse outputs are programmed just like the AB outputs.

• Good values to start with for the delay function are:

A = T+ 0.008000

B = A+ 0.000055

C = A+ 0.002965

D = C+ 0.000110

Here CD pulse (D = C+ 0.000110) is twice as long as AB (B = A+ 0.000055) pulse, separation between two pulses is much bigger than both of the pulse widths (C = A+ 0.002965), and 8ms after a trigger pulse (A = T+ 0.008000) the generator sends a step to the A output. (If a 55 microsec wide AB pulse does not work, then try to increase it and the CD pulse by a factor of two.)

The Output menu allows you to specify the high and low voltage levels for each of the steps and pulses, as well as the expected load on the outputs. You should not need to change anything in the Output menu. Here are the proper settings:

T: High-Z / TTL / Normal

AB 50 W / Var / Amplitude = 1V / Offset = -0.25 V

CD 50 W / Var / Amplitude = 1V / Offset = -0.25 V

A,B,C, and D aren't used, so their output settings don't matter.

The trigger output should go to the trigger input of the scope and the AB and CD output should be connected like this:

The output stages are such that if the output are connected, the voltage produced is the sum of the voltages programmed in for each channel: i.e. a baseline of -0.5 V with two pulses rising to +0.5 V. Thus the pulse sequence looks like the following. (Note that this drawing is not to scale. Typically the pulse separation is much longer than the length of either pulse.)

Note: CD pulse above is twice AB pulse width. You should adjust the pulse generator such that you know how to get these results.

THE SPIN ECHO EFFECT is induced by two radio frequency (RF) pulses, which tip the spin axes of the protons in a liquid sample in a constant magnetic field. [NOTE: the above two traces are an example of the results from spin echo effect signals. The top trace is the signal from the sample and the trace below it is the signal from the pulse generator.] The first pulse starts the precession of the protons in the sample, then the second pulse, which is twice as long as long as the first pulse, flips the plane of the magnetic moments and causes the echo pulse shown at number 6.

Once you have everything set up and you have a decent signal, try these:

A. Excitation level as a function of "pulse area."

An on-resonance radio frequency pulse will "tip" the average magnetization vector away from the applied DC magnetic field by and angle which is known as the "area" of the pulse. The area of the pulse is proportional to the integral over time of the RF field strength, and a pulse which tips the magnetization to exactly opposite its original value has and area of (it's called a -pulse). If the average magnetization starts polarized in the z-direction, then after a pulse of area , the component of the magnetization in the x-y plane will be proportional to . To see this experimentally, tune the oscillator as close to resonance as you can. On exact resonance there should be no wiggles in the free-induction decay. Next, vary the duration of the first pulse and record the height of the peak of the free-induction decay signal as a function of pulse duration. Note the duration of the pulses which correspond to maxima and minima

B. Pulse-Induced Transparency.

Immediately after a pulse, the magnetization has been tipped into the x-y plane, and it induces a voltage in the pickup coil as the nuclei precess. As the nuclei get out of phase with each other due to inhomogeneities in the magnetic field, we can think of each nucleus as being still tipped in the x-y plane and precessing, but pointing in random directions so that on average they induce no signal. The average magnetization is zero. If we now apply any pulse at all, we will not see a free induction decay (FID), because for every nucleus pointing in some direction, there is another nucleus pointing in the opposite direction, canceling its field. This collection of nuclei is transparent. Check this experimentally by applying a -pulse and then several milliseconds later another pulse. Does the FID signal after the second pulse depend more on the duration of the first or the second pulse? Why does the FID not completely disappear?

C. Spin Echoes.

Construct a pulse sequence which consists of first a -pulse, then several milliseconds later a -pulse. The nuclei that dephased after the first pulse should re-phase and produce an echo of the initial FID. Change the spacing between the two pulses and note the behavior of the echo. There are two characteristic times in this situation. T₂, the rate at which the average magnetization decays away due to dephasing can be measured from the FID, and T₁, the rate at which the magnetization of a single nucleus is lost can be measured from the decay of the echo as a function of pulse spacing.